This Is Why It's Difficult To Downshift Into First Gear
Changing gear can become an automotive art form when practiced and fully understood, forming a smooth relationship between ratios, engine speeds and differing shaft speed. Although first gear is generally only engineered to get a car off the line, hairpins and sharp corners can necessitate a change down from a higher ratio to the first cog.
If you have tried this before, you may have noticed just how difficult it can be to slot back into first gear, sometimes requiring a mechanically-unsympathetic shove. The most common situation that this occurs is when drivers are slowing down for a traffic light and pre-empt a downchange into first to get going again.
The problem is that the difference in ratios between second and first is large. So considering the job of a synchromesh is to bridge this gap, the synchromesh for first gear has to work a lot harder than in the rest of the transmission.
A synchromesh is almost like a small clutch that sits on the output shaft between gears, slowing or increasing the required gear’s relative speed to perform a perfect meshing of teeth within the transmission. So when trying to downshift to first, the relative speed difference between the output shaft and input shaft will be large compared to the other less-aggressive ratios.
Let’s take a 2016 Honda Civic’s transmission for example. The ratio for first gear is 3.6:1, meaning that for every 3.6 full rotations of the crankshaft, the gear rotates once. Second gear has a ratio of 2.1:1, third is 1.4:1, fourth 1:1, fifth 0.8:1 and sixth 0.7:1. As you can see, the difference in gear ratio gets smaller and smaller as you go up the gears, meaning the transition between each of them gets easier for the synchromeshes to perform.
The stiffness encountered when changing from second to first gear can also be experienced when committing the blasphemy of selecting second gear from fourth, skipping third gear. The difference in the speed of the input and output shafts of the transmission will be large, forcing the required synchromesh to work harder.
Solutions to help downshifting into first are double clutching and rev-matching. These will allow the transmission the chance to match the speed of the crankshaft, lessening the load applied to the synchromesh for first gear and aiding smoother shifts. Despite this, we wouldn’t recommend regular downshifts into first due to the strain it can put on the components within the transmission. So next time you feel that first gear is necessary to launch out of a corner, make sure you think about the required pedal inputs to perform a smooth shift.
Comments
“Shift to R for maximium speed!” - A non-car guy, year unknown
Would this also happen if I were to go to 1st from 17th?
You’re not supposed to shift down into first gear in normal driving anyway. It’s only there to set the car into motion, after that you can drive in second gear. Nearly any car can start in second gear as well, it just needs a certain lack of mechanical sympathy.
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I know this is kinda off-topic, but it’s so annoying when you’re driving in a city and the traffic is moving slowly and you try to use the 1st gear, but it’s too short, and when you change into second the car nearly stalls and you need to slip the clutch to keep it moving. Like c’mon people, step on the gas or stop completely.
Actually downshifting to first is only interesting in traffic and in that conditions I like to rev match it to keep the car smooth and quick
When stopping on the traffic lights I mostly press the clutch, change into first, but not depress it, untill I’m starting to move again. Downshifting in first is just raping the car.
yeah shifting down to 1st on the move is not the easiest, especially if the syncros are bad, but who uses 1st gear during normal driving anyway xD
I always rev match to downshift when i downshift to accelerate or to slow down the car but its super tricky when braking at the same time .. hardly possible.